feat(cluster): implement cluster detection and bridge notes discovery

Add automatic note clustering using density-based algorithm (DBSCAN variant)
and bridge notes detection for connecting different thematic clusters.

Features:
- NoteCluster, ClusterMember, BridgeNote, BridgeSuggestion models
- Clustering service with pgvector cosine similarity
- Bridge notes detection (notes connecting >=2 clusters)
- AI-powered suggestions for missing cluster connections
- /insights page with React Flow visualization
- Cron endpoint for automatic recalculation

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

memento-note/prisma/schema.prisma

@@ -7,7 +7,7 @@ generator client {
 datasource db {
   provider   = "postgresql"
   url        = env("DATABASE_URL")
-  extensions = [pgvector]
+  extensions = [vector]
 }
 
 model User {
@@ -50,6 +50,10 @@ model User {
   subscription          Subscription?
   usageLogs             UsageLog[]
   apiKeys               UserAPIKey[]
+  aiConsentLogs         AiConsentLog[]
+  noteClusters          NoteCluster[]
+  bridgeNotes           BridgeNote[]
+  bridgeSuggestions     BridgeSuggestion[]
 }
 
 model Account {
@@ -185,6 +189,8 @@ model Note {
   brainstormNoteRefs         BrainstormNoteRef[]
   outgoingLinks              NoteLink[]               @relation("SourceLinks")
   incomingLinks              NoteLink[]               @relation("TargetLinks")
+  clusterMemberships         ClusterMember[]
+  bridgeNote                 BridgeNote?
 
   @@index([isPinned])
   @@index([isArchived])
@@ -309,6 +315,7 @@ model UserAISettings {
   noteHistory          Boolean @default(false)
   noteHistoryMode      String  @default("manual")
   autoSave             Boolean @default(true)
+  aiProcessingConsent  Boolean @default(false)
   user                 User    @relation(fields: [userId], references: [id], onDelete: Cascade)
 
   @@index([memoryEcho])
@@ -317,6 +324,16 @@ model UserAISettings {
   @@index([preferredLanguage])
 }
 
+model AiConsentLog {
+  id        String   @id @default(cuid())
+  userId    String
+  consent   Boolean  @default(true)
+  ipAddress String?
+  userAgent String?
+  createdAt DateTime @default(now())
+  user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+}
+
 model NoteEmbedding {
   id        String   @id @default(cuid())
   noteId    String                         @unique
@@ -747,3 +764,76 @@ model FeatureFlag {
   metadata  String?
   updatedAt DateTime @updatedAt
 }
+
+// ===== CLUSTERING & BRIDGE NOTES =====
+
+model NoteCluster {
+  id              String   @id @default(cuid())
+  userId          String
+  clusterId       Int      // Cluster number for this user (0, 1, 2, ...)
+  name            String?  // Auto-generated cluster name
+  centroid        Unsupported("vector(1536)")? // Optional: centroid embedding for cluster
+  noteCount       Int      @default(0)
+  lastCalculated  DateTime @default(now())
+  createdAt       DateTime @default(now())
+  updatedAt       DateTime @updatedAt
+
+  user            User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@index([userId])
+  @@index([userId, clusterId])
+  @@unique([userId, clusterId])
+}
+
+model ClusterMember {
+  id              String   @id @default(cuid())
+  userId          String
+  noteId          String
+  clusterId       Int      // Cluster number
+  membershipScore Float    @default(0.0) // How strongly this note belongs to the cluster
+  isCentral       Boolean  @default(false) // Is this a central note in the cluster?
+  createdAt       DateTime @default(now())
+
+  note            Note     @relation(fields: [noteId], references: [id], onDelete: Cascade)
+
+  @@index([userId, clusterId])
+  @@index([noteId])
+  @@unique([noteId, clusterId])
+}
+
+model BridgeNote {
+  id                String   @id @default(cuid())
+  userId            String
+  noteId            String   @unique
+  bridgeScore       Float    // len(clusters_touched) / max_clusters
+  clustersConnected String   // JSON array: ["0", "2", "5"]
+  lastCalculated    DateTime @default(now())
+  createdAt         DateTime @default(now())
+  updatedAt         DateTime @updatedAt
+
+  note              Note     @relation(fields: [noteId], references: [id], onDelete: Cascade)
+  user              User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@index([userId])
+  @@index([bridgeScore])
+}
+
+model BridgeSuggestion {
+  id                String   @id @default(cuid())
+  userId            String
+  clusterAId        Int
+  clusterBId        Int
+  clusterAName      String
+  clusterBName      String
+  suggestedTitle    String
+  suggestedContent  String   // @default(dbgenerated)
+  justification     String   // Why this connection makes sense
+  isDismissed       Boolean  @default(false)
+  createdAt         DateTime @default(now())
+
+  user              User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@index([userId])
+  @@index([userId, isDismissed])
+  @@index([clusterAId, clusterBId])
+}